The present invention generally relates to the field of electromagnetic emissions testing. In particular the present invention relates to a method and apparatus for determining the likelihood of an electric field at a position and caused by electromagnetic emissions from a system a distance from the position being less than or more than a predefined electric field, where the system comprises a plurality of equipment units each of which individually radiate electromagnetic emissions.
There is a requirement for modern electrical and electronic equipment to meet a series of mandatory electromagnetic compatibility (EMC) requirements. One of the requirements is for the radiated electromagnetic emissions to be below a certain level in order to avoid interference to adjacent users of the radio frequency spectrum and other adjacent electrical and electronic equipment.
Certain electrical or electronic equipment is capable of being placed on a test facility whereupon the radiated electromagnetic emissions can be measured directly. Typically measurements are made at 3, 10, and 30 metres distance and the electronic fields at these distances are measured for vertical and horizontal polarities and for various radiation frequencies. The EMC regulations such as the European Union (EU) Directive 89/336/EEC which came into force on Jan. 1, 1996 sets field levels which must be met by both individual equipment units and systems comprising a plurality of equipment units.
Demonstrating compliance of equipment units is the responsibility of the body that makes the unit available for sale within the European market (either the manufacturer or the importer). Demonstrating the compliance of systems is the responsibility of the Systems Integretor or that body on whose behalf the system is bought into operation.
Whilst it is possible to place some equipment on a test facility and measure emissions whilst the equipment is operated, for larger systems and installations this may not just be impractical but also impossible. For example, the testing of the radiated emissions for a telephone exchange would require the complete telephone exchange to be placed on a test facility and it would be necessary to simulate typical operation of the exchange.
For this reason it is also possible in order to demonstrate compliance with the EMC regulations to generate what are termed Technical Construction Files (TCFs) for each system or installation. Under the terms of the EU Directive a TCF records all information relevant to the EMC performance of the item it concerns.
Using TCFs it is possible to demonstrate compliance with the EMC regulations by predicting the electromagnetic compatibility of the equipment and the present invention is concerned with a particular method of prediction that leasds the system owner to conflude that it complies with the protection requirements of the regulations.
In accordance with an aspect of the present invention there is provided a method of determining electromagnetic compatibility by predicting the likelihood of compliance with the radiated emissions standards. This is achieved by calculating the contribution to the electromagnetic field caused by emissions from equipment units forming the system and assigning a random relative phase to the components in order to build up a distribution of possible electromagnetic field values. This distribution can then be statistically analysed to determine the likelihood of compliance.
Thus for systems and installations which are comprised of large numbers of equipment units, the present inventor has realised that it is in principle possible to consider the field emitted from the system resulting from the combination of the emissions from the equipment units, if the following is known for each equipment unit: (I) the individually radiated electric field at the point of interest; (ii) the relative phase of the individually radiated electric fields at the point of interest. However, the relative phase of the emissions from the equipment units can not be accurately predicted. The inventor has overcome this problem by using a statistical technique whereby the relative phase of each contribution is randomised in order to determine a statistical distribution for the field emitted by the system. From this statistical distribution the likelihood of compliance with the standard can then be determined.
In one particular embodiment this statistical analysis technique comprises determining the cumulative probability for the electric field values and determining if, for the electric field threshold defined by the standard, the cumulative probability is above or below the threshold.
The statistical technique used in the present invention is considered to be a particularly valid one in view of the variability of emissions of the equipment due to both operating variations and manufacturing variations. In particular, in a publication The International Special Committee on Radio Interference, CISPR 22, 3rd Edition 1997-11, Section 7, Interpretation of CISPR Radio Distribution Limit, Sub-Section 7.1.2 it is stated xe2x80x9cThe significance of the limits for equipment shall be that, on a statistical basis, at least 80% of the mass produced equipment complies with the limits with at least 80% confidencexe2x80x9d.
In an embodiment of the present invention the relative phase is randomly selected in accordance with a predefined probability distribution of possible relative phases. Preferably, each possible relative phase has equal probability in the predefined probability distribution.
The individual electric field components generated by individual equipment units can be calculated based on the distances from the equipment units to the defined position and based on known electromagnetic emission properties of the equipment units. Such a known electromagnetic emission properties can either comprise a measured electric field at a distance from the equipment unit, or can be information which is provided with the equipment unit.
The electromagnetic emission from the units can be emitted at a plurality of frequencies and at various polarities. The method of determining the likelihood of compliance of the system with the standard can include determining the likelihood for each frequency and polarity e.g. vertical and horizontal polarities.
In an embodiment of the present invention the minimum possible compound electric field is determined using the individually calculated electrical field components and if the minimum possible compound electric field is greater than the predefined electric field, clearly the system cannot meet the standard and the statistical analysis is not carried.
The maximum possible compound electric field is determined using the individual calculated electric field components and if the maximum possible compound electric field is not greater than the predefined electric field, clearly the system cannot fail the standard and thus the statistical analysis need not be carried out. If the maximum possible compound electric field and the minimum possible compound electric field is less than the predefined electric field, is greater than the predefined electric field, the likelihood of the equipment complying with the standards must be determined.
Using the technique of the present invention, in an embodiment of the present invention it is possible to map around the system to provide xe2x80x9ccontoursxe2x80x9d of EMC compliance probability. This mapping may be important for example which considering an installation and its effect on its neighbours. For example, there may be highly sensitive equipment in the neighbourhood in one direction and thus in this direction the emissions will need to be reduced.
Using the method of the present invention it is thus possible to design a system which comprises a plurality of equipment units. Units can be arranged in dependence upon their emissions in order to meet the desired emission criteria. Such a method can be implemented on a computer to provide a graphical user interface allowing the user to move the units about within the model of the equipment.
The present invention can be implemented as a computer program operating on a standard computer and thus one aspect of the present invention provides a storage medium containing processor implementable instructions for controlling a processor to carry out the method. Another aspect of the present invention provides an electronic signal carrying computer code for instructing a processor to carry out the method.